1. Technical Field
The present invention is concerned with multilayer resist structures and methods of producing them which can be used in electronic device lithography to produce micrometer and submicrometer geometries.
2. Background Art
Electron beam lithography is one of the most promising methods for producing the micrometer and submicrometer geometries desired in electronic devices. However, a significant problem in electron beam lithography has been the effect of electron backscattering in the radiation-sensitive organic resist film, which has placed practical limitations on the resolution and line width control, due to proximity effects and resist thickness variations over the substrate topography. A lesser degree of radiation scattering may occur when the radiation source is X-ray.
The effects of electron scattering in an organic resist film on various substrates is described by Parikh et. al. in "Energy deposition functions in electron resist films on substrates", Journal of Applied Physics 50(2), February 1979, pp. 1104-1111.
The effect of multi-layer resists on electron scattering, and hence the proximity effect, is discussed by J.S. Greeneich in "Proximity Effects in Electron-Beam Exposure of Multi-Layer Resists" Proceedings Symposium, Electron and Ion Beam Science and Technology, 9th International Conference 282, Electrochemical Society (1980). This paper discusses the effect of using a thin layer (specifically 500 .ANG.) of aluminum as part of a multi-layer structure. The calculated data for the multi-layer resist indicate that when a thin layer of aluminum is used as an intermediate layer between upper and lower layers of polymeric resist, for a given dosage of radiation, the overall exposure of the upper layer of resist is increased. Thus, the reflection back (of radiation) from the aluminum layer is greater than the reflection back from a typical multi-layer structure comprising a second layer of resist atop a silicon substrate.
A method of using multilayer resist systems in electron beam lithography in order to create a high aspect ratio structure (with a large height-to-line width ratio) is described in U.S. Pat. No. 4,165,395 to Chang. A method for producing high resolution patterns in a thick layer of polymeric material is described in U.S. Pat. No. 4,362,598 to Griffing. Both of these patents refer to three layer resist structures wherein the upper and lower layers are polymeric material and the intermediate layer is a metal which is used as a mask for the lower resist layer during practice of the method. In Chang, the metallic intermediate layer acts as a mask during exposure of the lower resist layer to actinic radiation. In Griffing, the metallic intermediate layer acts as a mask during exposure of the lower resist layer to reactive ion etching in oxygen.